This invention relates to the field of polyamide resin blends and more particularly to novel blends which exhibit exceptionally high impact strength and crack resistance.
Polyamide resins are generally considered to be relatively tough materials. Based on favorable tensile strength, elongation and impact properties, polyamide resins have been found useful in a variety of mechanical applications. Although polyamides exhibit good impact strength in most respects, they are quite susceptible to crack propagation, as a result of which their notched impact strength can be rather poor. Because of such susceptibility, products constituted of polyamide are subject to brittle failure under service conditions where a product containing incipient cracks is subjected to moderate or heavy impacts.
Extensive research has been conducted for the purpose of developing polyamide resin blends of improved impact strength, and especially of improved resistance under notched impact test conditions such as those utilized in the notched Izod impact test, ASTM D-256. Thus, for example, Epstein U.S. Pat. No. 4,174,358 discloses multiphase thermoplastic compositions in which various branched and straight chain polymers having a particle size in a range of 0.01 to 1.0 microns are dispersed in a polyamide matrix resin and wherein the ratio of the tensile modulus of the polyamide matrix resin to the tensile modulus of the dispersed polymer is greater than 10 to 1. Although Epstein discloses a great variety of resin blends, the performance of those blends under notched Izod impact test conditions varies widely and, in some instances, the elongation is relatively low. Great variation in tensile modulus is also exhibited by the numerous individual blends disclosed by Epstein.
Mason et al. U.S. Pat. No. 4,160,790 discloses three component blends consisting of polyamide, an ethylene/acrylic acid copolymer and an ethylene ethyl acrylate copolymer. The blends of Mason et al. are said to exhibit synergistic improvement in impact resistance, but the highest notched Izod impact strength reported is 3.1 ft.-lbs./in.
Iwami et al. U.S. Pat. No. 4,105,709 discloses blends containing polyamide and a partially saponified copolymer of ethylene and an acrylic ester. The saponified copolymer is partially acidified so that, as blended with the polyamide, it contains units of ethylene, the acrylic or methacrylic ester, a salt of acrylic or methacrylic acid, and free acid. Iwami et al. are also concerned with the object of providing improved impact strength but the best of the results reported for Izod impact strength is 12.5 kg-cm/cm.
Thus, a need has remained for polyamide blends of improved notched impact strength which also exhibit favorable elongation, melt strength and other desirable mechanical properties. More particularly, there has been a need for such resin blends which are adapted for use in both molding and extrusion applications including the extrusion of polyamide film.